1. Field of the Invention
This invention relates to a solid state image sensor having a plurality of horizontal CCDs, and more particularly to a solid state image sensor in which the charge transfer between horizontal CCDs is improved.
2. Description of the Related Art
Recently, CCD image sensors have been widely used for home-use video cameras and the like and also used for broadcasting cameras. The advent of VHS systems such as the super VHS system (S-VHS system) for home-use video cameras and the HDTV system for the TV broadcasting increasingly requires the development of multi-picture element type solid state image sensors The number of picture elements is determined to be approximately 500 and 1000 in the S-VHS and HDTV systems, respectively, and therefore an attempt has been made to increase the integration density of picture elements in the horizontal direction so as to increase the total number of picture elements.
With an increase in the number of picture elements in the horizontal direction, the readout frequency of a signal output and the driving frequency of the horizontal CCD were increased so that the power for driving the CCD could be increased and the transfer efficiency in the horizontal CCD could be lowered in an image sensor using a single horizontal charge transfer CCD. In order to solve the above problems, a solid state image sensor having a plurality of horizontal CCDs and disclosed, for example, in an article ("1/2 inch 768.times.492 elements CCD image sensor" TV technical report, February 1986, TEBS 109 ED942) has been proposed.
In the conventional solid state image sensor using two horizontal CCDs, photodiodes acting as a photosensitive section are arranged in a matrix form on a semiconductor substrate and vertical charge transfer CCDs or vertical CCDs are arranged adjacent to respective vertical lines of photodiodes. The vertical CCD is driven by four-phase drive signals supplied via four sets of electrodes in a four-phase driving manner. Two horizontal CCDs constituting one stage for adjacent two lines of vertical CCDs are arranged in parallel, and two horizontal CCDs are respectively connected to output amplifiers. Further, two electrodes for matching the timings of charge transfer between the vertical CCD and horizontal CCD are disposed between the vertical CCD and horizontal CCD, and an electrode for transferring the charge between the horizontal CCDs is arranged between the two horizontal CCDs.
With the above solid state image sensor, signal charges stored in the photodiodes for a preset period of time are read out to the corresponding vertical CCDs and then transferred in a vertical direction. The signal charges transferred by the adjacent two vertical CCDs are transferred to one of the horizontal CCDs via the electrode which is supplied with drive signal .phi.VL. Those signal charges which are transferred along one of the vertical CCDs are further transferred to the other horizontal CCD via the electrode which is supplied with drive signal .phi.T. The signal charges which have been transferred along the adjacent vertical CCDs and supplied to the respective two horizontal CCDs are separately transferred in a horizontal direction along the respective horizontal CCDs and output from the output amplifiers.
The above solid state image sensor has the following problem. That is, the electrode lying on the channel acting as a path for the charges at the time of transferring charges between two horizontal CCDs is formed in a rectangular form, and if the electrode is formed of polysilicon, burrs or indentations of 2000 .ANG. are formed on the peripheral portion of the electrode. As a result, variation in the width (length in a horizontal direction) of each electrode becomes approximately 4000 .ANG. at maximum. The variation in the electrode width changes the potential of the channel through which signal charges pass. More specifically, when the electrode is made wider, the channel potential becomes higher, and when it is made narrower, the channel potential becomes lower. For example, assume that the the electrode width is L and projections of 2000 .ANG. are formed on both sides of the central portion thereof. Then, the electrode width of the central portion becomes (L+4000 .ANG.). As a result, the channel potential at the portion of 4000 .ANG. electrode width becomes higher than the channel potential at the other portion by .DELTA.V. In this case, if L is 4 .mu.m, .DELTA.V becomes approximately 50 mV. That is, .DELTA.V becomes higher than the thermal excitation voltage (the potential difference of a potential barrier which electrons can pass by thermal excitation) of approximately 25 mV. As a result, a portion (which is referred to as a potential pocket) at which the potential is higher than the other portion is partially created in the channel under the electrode, and the charge in the channel cannot be completely transferred. When picture element signals obtained by the incomplete charge transfer is displayed on the CRT, irregular vertical lines are displayed on the display screen, thus degrading the picture quality.